Guide for Protective Grounding of Power Lines
|Publication Date:||1 January 2005|
Protective grounding methods have often not kept pace with their increasing importance in work safety as the available fault current magnitudes grow, sometimes to as high as 100 kA, and as right-of-ways become more crowded with heavily loaded circuits, leading to growing problems of electric or magnetic induction. This guide has compiled state-of-the-art information on protective grounding practices employed by power utilities in North America.
The revision of the guide was undertaken to add information on the electrical hazards related to electric utility vehicles working adjacent to power lines. Electrostatic induction develops a voltage on the vehicle, its magnitude depending upon the vehicle's insulation from ground, the separation distance, and the voltage and current on the adjacent line. The hazards may include both a transient discharge current as the vehicle is grounded through a person's body and a steady-state capacitive current that may flow through the vehicle. The revision discusses the advantages of isolating the worksite by barricading the vehicle to protect the public from the electrical hazards.
The revision also includes factors important in sizing protective grounds. These factors are based on a review of current practices, technical information, and safety criteria. The factors are intended to protect electrical workers during work on de-energized transmission and distribution lines.
The primary purpose of factors in sizing protective grounds is to ensure that protective grounds utilized during de-energized work on transmission and distribution lines are sized to adequately protect workers from injury or electrocution. Protective ground sizing is required for the maximum magnitude and duration of current that may flow in a grounding system at a worksite. The current may be of short duration from accidental energization of the line or may be due to continuous current from magnetic induction by nearby energized circuits.
This revision addresses the size of protective grounds to carry fault currents or induced current at the worksite for the full duration of current. Determination of fault current magnitude considering the ac and dc offset components and the effect of X/R (X is reactance and R is resistance) ratios is discussed. Primary and backup relaying times are discussed because both items are important considerations for sizing protective grounds. The determination of currents induced by nearby energized circuits is also discussed as an essential consideration for sizing protective grounds.
The material requirements for protective grounding sets are discussed in detail. Recommended materials, ratings, component design and shape, jacket material, resistance, mechanical stress considerations, and details of multiple grounding systems are also covered.
Finally, a discussion of the practical use of protective grounds is covered. The preferred practice is to use a single protective ground at a worksite. Because a single protective ground cannot always be accomplished, it is permissible to parallel more than one grounding cable. The precautions required for multiple protective grounds used in parallel are discussed. This guide concludes with a presentation of in-service maintenance, inspection, and testing guidelines.
This guide was developed through the collaborative effort of an international group of volunteers with expertise in many disciplines. While this guide represents a consensus among this volunteer group, it is not the only view on the issues addressed herein. As with any guidance, use of this guide and the procedures and positions herein does not provide proof of or guarantee safety. Use and compliance with this IEEE guide are wholly voluntary.
This document provides guidelines for grounding methods to protect workers and the public from voltages that might develop in a jobsite during de-energized maintenance of overhead transmission and distribution lines.